Subterranean formations may be evaluated and/or monitored using information received from various measurement methods, such as methods for measuring seismic, acoustic, sonic, elastic and other properties downhole. Such methods typically include using receivers positioned at the surface or disposed in a borehole. An example of a borehole measurement technique utilizing seismic receivers includes the generation of a vertical seismic profile (VSP). VSP systems include seismic sources that generate seismic waves in an earth formation and seismic receivers positioned at selected downhole locations for receiving and measuring the seismic waves.
Some measurement systems, such as logging-while-drilling (LWD), VSP-while-drilling (VSP-WD) and other systems that measure properties such as seismic, acoustic, sonic and elastic properties downhole, utilize measurement tools that include sources or receivers disposed downhole without a wired or other physical connection to surface components. Such systems typically include clocks associated with downhole tools and with the surface components to record times that measurement signals are transmitted and received, so that the travel time of the signals can be measured. Accurate time synchronization between a surface clock that steers and/or monitors a source and clocks that steer data acquisition from receivers is essential for these measurement methods.
Current techniques for clock synchronization involve synchronizing the surface clock with downhole clocks prior to disposing a tool or sub in a borehole, and checking the time drift (caused by, for example, changes in temperature downhole) of the downhole clocks after the downhole clocks are pulled out of the borehole. These techniques generally do not include further synchronization while the sub or other tool is in the borehole. The downhole clocks required for such techniques thus must be maintained with a small error, requiring expensive and high power-consuming clocks. Such clocks require continuous powering for an extended period (typically about 3 to 4 days) before being disposed downhole, have a relatively small lifetime, and require an uninterruptible power supply during an entire survey. In addition, the power consumption of such downhole clocks is relatively large, and thus an external power source (such as a battery sub) is generally required, which limits the placement of the downhole receivers.